Staging: hv: Remove xen legacy code and check for Hyper-V

Removed legacy XEN layer from hypervisor setup, and made sure only
Hyper-V is Is a valid hypervisor to run on.

Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>.
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>